This is a study of the metabolism and function of the inositide phospholipids in human erythrocytes. These lipids are minor but metabolically active components of many types of cell membranes, and are relatively abundant in the red cell plasma membrane. They are implicated in maintenance of cell shape, protein cytoskeleton stabilization, and possibly calcium buffering, three interrelated phenomena that modulate the elasticity and mechanical stability of the cell and thus contribute critically to normal circulatory competence. The proposed research addresses two hypotheses; first, that inositide interconversions are a homeostatic mechanism that regulates the shape of the cell; and second, that the polyphosphoinositides form secondary binding sites for the cytoskeletal protein reticulum and hence stabilize membrane planarity. The function of the inositides in mature red cells will be examined from four standpoints. The specificity of their interactions with other cell components, particularly the proteins of the membrane reticulum, will be studied in cell fragments and model membranes. The calcium binding and selectivity of the polyphosphoinositides will be examined in model membranes, and the reciprocal effects of calcium and protein binding will be explored. The effects of the inositides and their metabolites, diagcylglycerol and phosphatidic acid, on calcium-mediated cell processes will be examined by direct and indirect manipulation of membrane composition. Conversely, the responses of inositide metabolism to agents and conditions that perturb membrane structure and cell morphology will be examined in intact cells and resealed ghosts. Overall, the research is designed to explore the connections between phospholipid metabolism and the mechanical characteristics of the cell membrane.